As means for measuring the brain functions of a human body, there has been developed an optical topographic technique that analyzes the intensity of a return near-infrared light which has been applied to a part of the head and then passes through the head to two-dimensionally display the blood distribution of the cerebral cortex. The above technique proves the correspondence of the motor functions of a human body and the localized regions of the brains, to thereby find a clue as to a new psychiatric treatment. Also, in recent years, there has been advanced the development of an interface technique that directly controls an external device such as a computer, a game machine, or an environmental control unit by the aid of a measurement signal from the brain with the use of localized brain functions. For example, JP-A-1997-149894 has proposed a method of measuring the intensity of head transmitted light by the aid of a living body optical measuring device, calculating the quantity of oxygenated and reduced hemoglobin by means of an arithmetic device, and driving the external device by the aid of calculated data. JP-A-2000-172407 has proposed a method of determining the history of changes in measurement signals that have been obtained from the living body optical measuring device by the aid of the arithmetic device, a storage device, or a control unit, and applying the discrimination results to a specific rule to change over the channels of a television receiver. Also, JP-A-2002-172106 has proposed an interface technique of bringing an optical irradiator and a photodetector in contact with the skin of a body to be examined, and controlling an object of a screen according to the intensity of the obtained light signals.